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The Lake Sammamish story
Lake Sammamish in King County is a significant and scenic lake that attracts over a million visitors annually for activities such as fishing, swimming, boating, and water skiing.
The lake supports diverse species, including salmon, warm water fish, birds, and other wildlife. As the sixth largest lake in Washington, it is designated a natural resource of statewide importance under the Shoreline Management Act.
Historical and ongoing pollution issues
In 1968, a project costing $4.5 million (equivalent to roughly $40 million in 2024) diverted sewage away from the lake, reducing phosphorus inputs by 35%. Over the next five to ten years, this led to a significant decrease in phosphorus concentrations and improved water clarity. However, as urban development in the watershed increased, phosphorus runoff increased, causing pollution levels to climb once again.
Impact of Urban Development on Phosphorus in the Water
Rapid urbanization in King County transformed the lake's forested watershed into urban and suburban areas. This change impacts the quantity, quality, and timing of water flows into the lake, potentially impacting its water quality.
There is no single source of pollution coming into the lake. Phosphorus, a chemical of concern, comes from everywhere. It is generated by almost every land use activity in the watershed: forests, logging practices, farms, homes, gardens, construction sites, natural erosion processes, stormwater runoff, commercial developments, car washing, septic tanks, and more. This type of pollution generated by multiple sources is called non-point source pollution.
How Urbanization Changes the Flow of Water
The conversion of forests to impervious surfaces, like roads and buildings, reduces the soil’s ability to store water, increasing runoff rates. This leads to higher wet weather flows, causing more erosion and sediment transport into the lake, while reducing summer low flows and harming in-stream habitats. Lake Sammamish experiences the cumulative impacts of these land use changes on its water quality.
Restoration Efforts and Ongoing Challenges
In 1995, the Lake Sammamish Initiative was launched, and a citizen's task force developed the 1996 Lake Sammamish Water Quality Management Plan. The plan identified phosphorus pollution sources and strategies to reduce them, setting annual goals for phosphorus concentration, chlorophyll a levels (a measure of algae), and water transparency. Efforts include forest conservation, reducing phosphorus use, improving stormwater facilities, better erosion control enforcement, and targeted projects to address specific pollution sources.
Despite these efforts, challenges remain. Urbanization continues to impact water flows and quality, and historic mining operations still affect the lake's hydrology. Ongoing initiatives aim to protect Lake Sammamish's ecological health and public benefits, adapting strategies as knowledge and conditions change.
In August 1999, the area incorporated into the City of Sammamish. King County continues to monitor the lake water quality to understand the lake's response to management actions and continued impacts from non-point pollution.